Grim Initiate

To amass [subtype] N means “If you don’t control an Army creature, create a 0/0 black [subtype] Army creature token. Choose an Army creature you control. Put N +1/+1 counters on that creature. If it isn’t a [subtype], it becomes a [subtype] in addition to its other types.”

A player “amassed” after the process described in rule 701.44a is complete, even if some or all of those actions were impossible.

The phrases “the Army you amassed” and “the amassed Army” refer to the creature you chose, whether or not it received counters.

Some older cards were printed with amass N without including a subtype. Those cards have received errata in the Oracle card reference so that they read “amass Zombies N.”

First strike is a static ability that modifies the rules for the combat damage step. (See rule 510, “Combat Damage Step.”)

If at least one attacking or blocking creature has first strike or double strike (see rule 702.4) as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.

Giving first strike to a creature without it after combat damage has already been dealt in the first combat damage step won’t preclude that creature from assigning combat damage in the second combat damage step. Removing first strike from a creature after it has already dealt combat damage in the first combat damage step won’t allow it to also assign combat damage in the second combat damage step (unless the creature has double strike).

Multiple instances of first strike on the same creature are redundant.